Probing the Structural/Electronic Diversity and Thermal Stability of Various Nanocrystalline Powders of Gallium Nitride Gan
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6816 Chem. Mater. 2008, 20, 6816–6828 Probing the Structural/Electronic Diversity and Thermal Stability of Various Nanocrystalline Powders of Gallium Nitride GaN Mariusz Drygas,† Zbigniew Olejniczak,‡ Ewa Grzanka,§ Miroslaw M. Bucko,| Robert T. Paine,⊥ and Jerzy F. Janik*,† Faculty of Fuels and Energy and Faculty of Materials Science and Ceramics, AGH UniVersity of Science and Technology, al. Mickiewicza 30, 30-059 Krakow, Poland, Institute of Nuclear Physics, Polish Academy of Sciences, ul. Radzikowskiego 152, 31-342 Krakow, Poland, Institute of High Pressure Physics, Polish Academy of Sciences, ul. Sokolowska 29/37, 01-142 Warszawa, Poland, and Department of Chemistry, UniVersity of New Mexico, Albuquerque, New Mexico 87131 ReceiVed March 5, 2008. ReVised Manuscript ReceiVed July 29, 2008 Nineteen different gallium nitride nanopowders were prepared with four distinct synthesis methods under a wide range of experimental conditions. Variations of synthetic details were aimed at making diverse GaN powders, mainly, from a viewpoint of their structural and thermal stability properties. The supplemental application of powder XRD and 69Ga MAS NMR spectroscopic methods enabled a coherent insight into both the long-range structural order and electronic features, respectively, supporting the specific Knight shift effect displayed in the NMR spectra of the powders. Thermogravimetric measurements under a neutral gas atmosphere were conducted to probe stability ranges of the powders. The combined results support a broad diversity of GaN nanopowders that require supplemental characterization methods to satisfactorily define their complex structural, compositional, and electronic properties evolved during high-temperature syntheses or thermal annealing. Introduction stock, a limiting factor in successful optimization of the sintering process. The utilization of powder forms of many new materials including semiconducting gallium nitride GaN is far from Powder XRD has been a key method for structure satisfactory despite significant progress in the development elucidation of GaN crystalline powders. However, estima- of relevant synthetic methods.1 One of the attractive ap- tions of even such common parameters as average crystallite plications to consider is powder sintering into mechanically sizes and lattice parameters can be severely misinterpreted robust and machinable ceramics that could be a material of for nanopowders if standard calculation tools that have been 3 choice, for instance, for an economical replacement of GaN derived for microcrystalline systems are applied. In addition, single-crystal supports in optoelectronics. Preliminary reports gallium nitride is known to exhibit a range of defected on the high-temperature high-pressure sintering of nano-GaN hexagonal structures in the nanosized region. Furthermore, powders highlighting selected follow-up applications of the GaN can tolerate a rather undefined extent of nitrogen resulting ceramics have been published elsewhere.2 The deficiency vs the ideal 1/1 Ga/N ratio, with the latter feature crucial conclusion from these efforts based on examination having an undetermined impact on grain crystallinity. All of the GaN ceramic quality was a challenging need for the this makes standard XRD data for nanocrystalline GaN reproducible and unequivocally characterized powder feed- powders highly equivocal. The ambiguity of the XRD data for a limited number of * Corresponding author. E-mail: [email protected]. 69 † such powders has been substantiated by supplemental Ga Faculty of Fuels and Energy, AGH University of Science and Technology. 71 ‡ Institute of Nuclear Physics, Polish Academy of Sciences. or Ga MAS NMR measurements that in principle probe § Institute of High Pressure Physics, Polish Academy of Sciences. | short-range chemical environments of gallium centers in both Faculty of Materials Science and Ceramics, AGH University of Science 4-6 5 and Technology. hexagonal GaN and cubic GaN powders, as stressed ⊥ University of New Mexico. (1) (a) Janik, J. F. Powder Technol. 2005, 152, 118. (b) Gubin, S. P.; Kataeva, N. A.; Khomutov, G. B. Russ. Chem. Bull., Int. Ed. 2005, (3) (a) Palosz, B.; Grzanka, E.; Gierlotka, S.; Stel’makh, S.; Pielaszek, 54 (4), 827. (c) Trindade, T.; O’Brien, P.; Pickett, N. L. Chem. Mater. R.; Lojkowski, W.; Bismayer, U.; Neuefeind, J.; Weber, H. P.; Palosz, 2001, 13, 3843. W. Phase Transitions, Part B 2003, 76 (1-2), 171. (b) Petkov, V.; (2) (a) Stelmakh, S.; Swiderska-Sroda, A.; Kalisz, G.; Gierlotka, S.; Gateshki, M.; Choi, J.; Gillan, E. G.; Ren, Y. J. Mater. Chem. 2005, Grzanka, E.; Palosz, B.; Drygas, M.; Janik, J. F.; Paine, R. T. 15, 4654. (c) Tamas, U.; Gubicza, J. Z. Kristallogr. 2007, 222, 114. Proceedings of the International Conference on Nanoscience and (4) (a) Han, O. H.; Timken, H. K. C.; Oldfield, E. J. Chem. Phys. 1988, Technology, Basel, Switzerland, July 30-Aug 4, 2006; Institute of 89 (10), 6046. (b) Jung, W.-S.; Park, C.; Han, S. Bull. Korean Chem. Physics Publishing: Philadelphia, PA, 2006; Poster 1399. (b) Grzanka, Soc. 2003, 24 (7), 1011. (c) Jung, W.-S. Mater. Lett. 2006, 60, 2954. E.; Stelmakh, S.; Gierlotka, S.; Swiderska-Sroda, A.; Kalisz, G.; Palosz, (d) Schwenzer, B; Hu, J; Seshadri, R,; Keller, S; DenBaars, S. P.; B.; Drygas, M.; Janik, J. F.; Paine, R. T. Presented at the European Mishra, U. K. Chem. Mater. 2004, 16, 5088. (e) Schwenzer, B.; Hu, Powder Diffraction Conference, EPDIC-10, Geneva, Switzerland, Sept J.; Wuc, Y.; Mishra, U. K. Solid State Sci. 2006, 8, 1193. 1-4, 2006; Z. Kristallogr., 2007, 26, MS11-P92. (c) Borysiuk, J.; (5) (a) Yesinowski, J. P.; Purdy, A. P. J. Am. Chem. Soc. 2004, 126, 9166. Caban, P.; Strupinski, W.; Gierlotka, S.; Stelmakh, S.; Janik, J. F. (b) Jung, W.-S.; Han, O. H.; Chae, S.-A. Mater. Chem. Phys. 2006, Cryst. Res. Technol. 2007, 42 (12), 1291. 100, 199. 10.1021/cm800645q CCC: $40.75 2008 American Chemical Society Published on Web 10/16/2008 DiVersity of Stability of Nanocrystalline GaN Powders Chem. Mater., Vol. 20, No. 21, 2008 6817 originally in studies by W.-S. Jung et al.4b,c and B. Schwenzer wders. They were prepared with several processing condi- et al.4d,e In the case of hexagonal GaN, especially for GaN tions by using four different synthesis methods thus providing powders obtained in the 1000-1100 °C range from am- an unparalleled set of materials. Powder XRD and 69Ga MAS monolysis of oxygen-bearing gallium precursors, the well- NMR spectroscopy occasionally supplemented with XPS defined hexagonal pattern for h-GaN in its XRD scan has determinations were used for structural assignments. In this been associated with a usually minor resonance at ca. 330 regard, if the peak resolution is not of major concern ppm in the respective gallium NMR spectrum. In contrast, (significant distances between resonance frequencies), both a major broad NMR line at ca. 400-450 ppm has been linked the 69Ga and 71Ga NMR experiments provide qualitatively by a few researchers to a structurally undefined nitrogen matched and quantitatively comparable results while the deficient phase or impurity (O. H. Han,4a W.-S. Jung,4b,c B. observed peak positions usually vary by a few parts per Schwenzer4d,e). In only one study known to us, a few high- million. Because the resonance frequencies in GaN for the temperature made Ga15N powders have been shown by XRD observed species differ by some plus 100 ppm, even with to display a rather unusual double hexagonal pattern, the significant peak broadness one gets satisfactory data suggesting the existence of two different h-GaN phases.4d,e resolution from the 69Ga NMR experiment. TGA/DTA Some of these powders exhibited a sharp resonance at 330 studies under an UHP helium atmosphere were carried out ppm in the 71Ga MAS NMR spectrum accompanied by a to evaluate powder stabilities at elevated temperatures. For broad resonance with shoulders in the 350-500 ppm range. several samples, residual oxygen contents were also deter- Similarly shaped resonances were also observed in the mined to estimate the completeness of nitridation. respective 15N MAS NMR spectrum. The existence of two hexagonal phases and the presence of two sets of the Experimental Section correlated multinuclear NMR resonances were proposed as an argument for the crystalline (hexagonal) character of the Preparation. Four distinct synthesis routes were applied to make distinct N-deficient phase. GaN powders. (i) A two-stage aerosol-assisted method utilized · x In an alternative breakthrough proposal, the Knight shift aqueous or DMF solutions of gallium nitrate Ga(NO3)3 H2O (Strem Chemicals).8 First, an aerosol mist of the solution was due to conduction electrons in a semiconductor has been injected into a ceramic tubular reactor where it mixed with an proposed by Yesinowski and co-workers to account for the ammonia-rich atmosphere at 1000-1100 °C under specific process second high-frequency peak shifting and broadening in the conditions to yield a raw aerosol powder with an average composi- gallium MAS NMR spectra for at least some GaN powders, tion GaxOyNz. This stage was followed by pyrolysis of the raw especially those intentionally doped and/or containing im- powders at a selected temperature in the 900-1000 °C range under purities even at low levels of a few tenths of a percent.5a,6 an ammonia flow, 0.2 L/min, producing gray-yellowish materials. This well-substantiated approach stresses the importance of It is worth noting that the pyrolysis experiments resulted also in electronic factors due to dopant (intentional or unintentional) the formation of substantial quantities of black deposits at the related conduction electrons over purely short-range chemical reactor’s cold exit. In two separate cases, the first stage was carried ° environment effects and is also consistent with the single out under a neutral nitrogen atmosphere at 400 or 1050 C to yield pattern for h-GaN in most of the corresponding XRD spectra. aerosol-generated Ga2O3 as raw powders that were later pyrolyzed in the second stage under an ammonia flow resulting in severely It should not be surprising that the structural ambiguities agglomerated GaN bodies.